The present invention relates to an ignition apparatus for an internal combustion engine which is able to fire cylinders of the engine without fail irrespective of variations in the number of revolutions per minute of the engine as well as in the voltage of a power supply to the ignition apparatus. More specifically, it relates to an ignition apparatus of the character as above described which is simple in construction and high in operational reliability.
As generally recogized, with internal combustion engines such as automotive gasoline engines in which a plurality of cylinders are operated through four cycles including an intake stroke, a compression stroke, a combustion stroke and an exhaust stroke, an air/fuel mixture in each cylinder is compressed by a piston and a spark is generated by a spark plug at an optimum ignition timing for proper combustion to generate output power. At the time of ignition, in order for the explosive force generated by the combustion of the mixture in each cylinder to act as a force for pushing down a corresponding piston in an efficient manner, it is critical to generate a spark of sufficient energy at a proper crank position of each cylinder.
Accordingly, with this type of internal combustion engine, for the purpose of properly controlling the order of fuel injections by injectors, the timing for power supply to the ignition coil and the ignition timing for each cylinder, it is necessary to generate an ignition signal is synchrony with the rotation of the engine and in dependence upon the number of revolutions per minute of the engine as well as other various driving conditions so that the conduction of the ignition coil and the firing timing for each cylinder are optimally controlled. In order to generate a proper ignition signal at proper timings, electromagnetic pickup coils are employed, for example, which generate an AC pulse signal in correspondence with the rotation of the engine crankshaft.
The ignition signals produced by the electromagnetic pickups are generated at timings corresponding to a certain predetermined crank angle of each cylinder and have a peak level corresponding to the number of revolutions per minute of the engine. The ignition signals thus produced are compared with a reference voltage in a comparison circuit and waveform shaped to provide a signal having a rectangular waveform which is then utilized to turn on and off a switching means such as, for example, a power transistor for controlling the power supply to the ignition coil.
Even with the use of an ignition signal thus waveform shaped, however, it is impossible to sharply raise or increase the primary current supplied to a primary winding of the ignition coil due mainly to an inductance component of the ignition coil. On the other hand, the discharge energy of the spark plug is determined by the primary current at the time when the power supply to the ignition coil is cut. As a result, a prescribed conduction time for the ignition coil is required for proper combustion of a mixture in each cylinder. That is, too early starting of power supply results in wasteful power consumption, whereas too late starting of power supply often results in misfiring. Accordingly, in order to ensure a proper conduction time, the timing for starting conduction should be appropriately changed in response to the number of revolutions per minute of the engine.
Further, in an early period of engine starting, the source voltage of a battery, which is usually 12 volts, generally drops to about 6 to 10 volts. As a result, in order to ensure a sufficient primary winding current for the ignition coil, it is necessary to lengthen the conduction time and for the purpose of compensating for a possible drop in the source voltage, the timing for starting power supply should be advanced.
In order to meet these requirements, it was proposed in the past that the voltage level of the ignition signal and the voltage level of the reference voltage employed for waveform shaping should be changed in dependence upon the number of revolutions per minute of the engine and the source voltage.
FIG. 5 illustrates a typical example of such a conventional ignition apparatus for an internal combustion engine as described in Japanese Patent Laid-Open No. 54-43433. The apparatus illustrated includes an electromagnetic pickup coil 1 for generating an ignition signal V.sub.I in the form of an AC pulse which has a pulse width corresponding to the number of revolutions per minute of the engine in synchrony with the rotation thereof. The elctromagnetic pickup coil 1 may be a coil having one end disposed in a spaced opposing relation with the crankshaft (not shown) which is provided on the outer periphel surface thereof with a plurality of magnetic elements which are disposed at circumferentially equal intervals.
A comparator 2 compares the ignition signal V.sub.I from the electromagnetic pickup coil 1 with a reference voltage V.sub.R to provide a waveform shaped signal V.sub.IR having a rectangular waveform. An amplifier 3 properly controls or amplifies the voltage level of the output signal V.sub.IR from the comparator 2 which is fed to a switching means 4. The switching means 4 comprises a pair of first and second power transistors 4a, 4b coupled with each other in a two-staged manner. The first transistor 4a has a base connected to the output terminal of the amplifier 3, a collector coupled to a collector of the second transistor 4b and an emitter coupled to a base of the second transistor 4b which has an emitter connected to ground through a resistor 4c. The collector of the second transistor 4b is connected to one end of a primary winding of an ignition coil 6. The other end of the primary winding is connected to a battery 5 having a source voltage of V.sub.B volts (e.g., 12 volts). The ignition coil 6 includes the primary winding and a secondary winding which has one end thereof connected with the other end of the primary winding. A spark plug 7 is connected between the other end of the secondary winding of the ignition coil 6 and ground for generating a spark of a magnitude proportional to the primary current I.sub.I flowing in the primary winding at the time when the primary current is cut off.
An integration curcuit 10 integrates the ignition signal V.sub.I from the pickup coil 1 and generates a voltage. A representative of the number of revolutions per minute of the engine. The integration circuit 10 includes a diode 11 having an anode thereof connected to one end of the electromagnetic pickup coil 1 and a cathode thereof coupled to one end of a capacitor 12 which is grounded at the other end thereof, and a resistor 13 coupled in parallel with the capacitor 12. An amplifier 14 properly controls or amplifies the level of the voltage representative of the engine rpm at a node between the diode 11 and the capacitor 12. A pair of serially connected voltage-dividing resistors 15, 16 are connected between the output terminal of the amplifier 12 and ground for appropriately dividing the output voltage of the amplifier 14. The output voltage of the amplifier 14 thus divided by the resistors 15, 16 (i.e., the voltage across the resistor 16), which is designated by reference character B, is provided at a node between the resistors 15, 16. A comparator 17 has a positive or non-inverted input terminal connected to a node between the grounded emitter of the two-staged transistor couple 4 and the resistor 4c so as to be imposed upon by a voltage E.sub.I across the resistor 4a developed by the primary winding current I.sub.I flowing through the primary winding of the ignition coil 6 and the two-staged transistor couple 4, and a negative or inverted input terminal connected to a power supply 18 so as to be supplied with a reference voltage E.sub.R. The comparator 17 compares the primary winding voltage E.sub.I with the reference voltage E.sub.R and generates an output signal F is E.sub.I &gt;E.sub.R. The comparator 17 has an output terminal connected through a resistor 19a to the base of a transistor 19 which has a collector connected to the anode of the diode 11 of the integration circuit 10, and an emitter connected to ground. The comparator 17, the power supply 18 and the transistor 19 constitute a control circuit for controlling the output voltage A of the integration circuit 10 representative of the engine rpm in such a manner that the voltage A is reduced when the voltage E.sub.I corresponding to the primary current I.sub.I reaches the reference voltage E.sub.R.
A bias circuit 20 is connected to the other end of the electromagnetic pickup coil 1 for generating a bias voltage V.sub.IB corresponding to the divided rpm voltage B across the resistor 16. The bias ccircuit 20 acts as a level control means for properly changing the voltage level of the ignition signal V.sub.I. The bias circuit 20 includes a transistor 21 having a grounded collector and being driven by the divided rpm voltage B, a first power supply 22 having a constant voltage interposed between the battery 5 and the emitter of the transistor 21, a transistor 23 having a collector and a base connected to the opposite ends of the first power supply 22, respectively, so as to be thereby driven, a resistor 24 connected between the emitter of the transistor 23 and the other end of the pickup coil 1, a second power supply 25 having a constant voltage connected to the battery 5, a group of diodes 26 interposed between the second power supply 25 and ground with their polarities directed normally, a transistor 27 having a collector and a base connected to the opposite ends of the second power supply 25, respectively, a resistor 28 interposed between the emitter of the transistor 27 and the pickup coil 1, and a resistor 29 interposed between the emitter of the transistor 27 and ground.
An on-level setting circuit 30 operates to set a reference voltage V.sub.R in the form of an on-level reference voltage with which the output voltage V.sub.I of the pickup coil 1 is compared by the comparator 2. The circuit 30 generates an on-level reference voltage which varies in dependence upon the source voltage V.sub.B of the battery 5. The circuit 30 includes a constant voltage supply 31 connected to the battery 5, a group of diodes 32 connected to the constant voltage supply 31 with their polarities directed normally, a transistor 33 having a collector and a base connected to the opposite ends of the constant voltage supply 31, respectively, so as to be thereby driven, a resistor 34 for dividing the source voltage V.sub.S of the battery 5 to provide a partial or divided voltage V.sub.SS, a transistor 35 having a grounded collector, an emitter commonly connected to the cathodes of the grouped diodes 32 and a base connected to one end of the resistor 34 so as to be driven by the divided voltage V.sub.SS, a group of three transistors 36 connected between the battery 5 and the resistor 34, a group of two transistors 37 in the form of a so-called current mirror circuit connected between the battery 5 and ground as well as between the group of the transistors 36 and ground, a group of two transistors 38 in the form of a so-called current mirror circuit connected between the battery 5 and ground as well as between the group of transistors 37 and ground, and a Zener diode 39 interposed between the battery 5 and ground. A resistor R1 is interposed between the Zener diode 39 and the battery 5. A resistor R2 is interposed between the resistor R1 and the group of transistors 38. A resistor R4 is interposed between the group of transistors 37 and ground. A resistor R5 is interposed between a junction between the resistors R1, R2 and a junction between the group of transistors 36 and the resistor 34.
The emitter of the transistor 33 is commonly connected to one end of a resistor 44 which is grounded at the other end thereof, and to the negative or inverted input terminal of the comparator 2 so that a current having a magnitude proportional to the base-emitter voltage across the transistor 33, which is determined by the group of diodes 32 and the transistor 35, flows through the transistor 33, developing a reference voltage V.sub.R across the resistor 44 which is fed to the negative input terminal of the comparator 2.
An off-level setting circuit 40 operates to set a reference voltage V.sub.R in the form of an off-level reference voltage, which is lower than the on-level reference voltage, so as to provide hysteresis. Thus, the circuit 40 outputs the off-level reference voltage as the reference voltage V.sub.R to the negative input terminal of the comparator 2. The circuit 40 comprises a constant current supply 41 connected to the battery 5, a group of serially connected diodes 42 connected between the constant current supply 41 and ground with their polarities directed normally, a transistor 43 having a collector and a base connected to the opposite ends of the constant current supply 41, respectively, so as to be thereby driven, a resistor 44 connected between the emitter of the transistor 43 and ground, a transistor 45 having a grounded emitter, a base connected to the output terminal of the comparator 2, and a collector connected to the base of the transistor 33, and a resistor 46 connected between the output terminal of the comparator 2 and the base of the transistor 45. The collector and the emitter of the transistor 43 are connected to the collector and the emitter, respectively, of the transistor 33 in the on-level setting circuit 30. The resistor 44 has one end thereof commonly connected to the emitters of the transistors 33, 43. Accordingly, a current, which is proportional to a base-emitter voltage of the transistor 43 determined by the group of diodes 42, flows through the transistor 43 so that a voltage across the resistor 44 is thereby developed and supplied to the negative input terminal of the comparator 2.
The transistor 45 is driven to turn on upon rising (i.e., a rising edge) of the ignition signal V.sub.IR from the electromagnetic pickup coil 1 so that the current from the constant current supply 31 is bypassed to turn off the transistor 33.
The operation of the above-described conventional ingition apparatus for an internal combustion engine will now be described in detail while referring to a waveform diagram illustrated in FIG. 6.
First, the electromagnetic pickup coil 1 generates, in synchrony with the rotation of the engine crankshaft, an ignition signal V.sub.I having a peak level corresponding to the number of revolutions of the crankshaft. The ignition signal V.sub.I is fed to the positive or non-inverted input terminal of the comparator 2 where it is compared with a reference voltage V.sub.R fed to the negative input terminal thereof and waveform shaped into a rectangular pulse signal V.sub.IR containing rectangular pulses each of which has a rising edge and a falling edge. The thus shaped ignition signal V.sub.IR is properly amplified by the amplifier 3 and fet to the base of the first transistor 4a of the two-staged transistor couple 4 to drive the second transistor 4b thereof into a conductive state. Thus, a primary current I.sub.I begins to flow through the primary winding of the ignition coil 6 which is then cut off upon falling (i.e., a falling edge) of the shaped ignition signal V.sub.IR. As a result, the spark plug 7 connected to the secondary winding of the ignition coil 6 generates a spark, thus firing a cylinder at a predetermined proper timing.
In this connection, it is supposed that the bias voltage V.sub.IB produced by the biasing circuit 20, which is determined by the constant current supply 25, the group of diodes 26, the transistor 27 and the resistor 29, be set to be at a prescribed constant level sufficient to operate the comparator 2.
On the other hand, the integration circuit 10 integrates the ignition signal V.sub.I from the pickup coil 1 to perform frequency to voltage conversion to provide a rpm voltage representative of the number of revolutions per minute of the engine. Specifically, each time the pickup coil 1 generates an ignition signal V.sub.I, the capacitor 12 is charged or discharged through the resistor 13. Therefore, as the number of revolutions per minute of the engine increases to increase the frequency of the ignition signal V.sub.I, the rate of charging the capacitor 12 becomes greater than the rate of discharging, thus increasing the rpm voltage A.
As a consequence, the divided rpm voltage B output from the amplifier 14 becomes higher, increasing the base voltage of the transistor 21 in the bias circuit 20. Accordingly, a current begins to flow from the constant current supply 22 to the base of the transistor 23, generating a voltage across the resistor 24. As a result, the bias voltage V.sub.IS is changed such that it is set by the transistor 23 and the resistor 24 and increases with the increasing rotational speed of the engine. Namely, the greater the rotational speed of the engine, the higher becomes the voltage level of the ignition signal V.sub.I from the pickup coil 1, so that the rising of each pulse of the shaped ignition signal V.sub.IR becomes more rapid or sharper, advancing the timing of starting the current supply to the primary winding of the ignition coil 6.
Further, the emitter voltage of the transistor 27 in the bias circuit 20, though it is superposed on the bias voltage V.sub.IB, remains constant within the normal operating voltage range of the transistor 27 since the base voltage thereof determined by the constant current supply 25 and the group of diodes 26 is held constant. Accordingly, the bias voltage V.sub.IB, which is determined by the transistor 27 and the resistor 29, acts to raise the voltage level of the ignition signal V.sub.I by a prescribed level, thus operating the comparator 2 without fail.
On the other hand, the on-level setting circuit 30 outputs a low-level reference voltage V.sub.R when the source voltage V.sub.B of the battery 5 is low whereas it outputs a high reference voltage V.sub.R when the source voltage V.sub.B is high. As a result, it becomes possible to make a pulse of the shaped ignition signal V.sub.IR rise not only at an early or advanced timing to ensure a sufficient conduction time for the ignition coil 6 when the source voltage V.sub.5 of the battery 5 is low, but also at a late or retarded timing to avoid wasteful power consumption when the source voltage V.sub.B is high. Specifically, the one-level reference voltage V.sub.R is determined by the following formula, EQU V.sub.R =V.sub.SS +3V.sub.F -V.sub.F
where V.sub.F is the voltage across each of the diodes 32, the voltage across the transistor 33, and the voltage across the transistor 35.
Here, assuming that the resistances of the resistors R1 through R5 and 34 are R.sub.1 through R.sub.6, respectively, and the divided voltage at the node between the resistors R1, R2 is V.sub.A, a divided voltage V.sub.SS across the resistor 34 in the case of a low source voltage V.sub.B of the battery 5 is expressed as follows. EQU V.sub.SS =V.sub.A .times.R.sub.6 /(R.sub.5 +R.sub.6)+R.sub.6 [V.sub.B -R.sub.3 (V.sub.A -V.sub.F)/R.sub.2 -V.sub.F ]/R.sub.4
In this connection, assuming that the currents through the resistors R2, R3 are i.sub.2, i.sub.3, respectively, the following equation is established. EQU (V.sub.A -V.sub.F)/R.sub.2 =i.sub.2 =i.sub.3
As a result, assuming that the currents through the collectors of the pair of opposed transistors of the grouped transistors 36, which have their bases coupled with each other, are i.sub.4, i.sub.6, respectively, the following equation is established. EQU [V.sub.B -R.sub.3 (V.sub.A -V.sub.F)/R.sub.2 -V.sub.F ]/R.sub.4 =i.sub.4 =i.sub.6
On the other hand, the divided voltage V.sub.BB across the resistor 34 in the case of a high source voltage V.sub.B of the battery 5, in which the Zener diode 39 is broken down, is expressed as follows; EQU V.sub.BB =V.sub.Z .times.R.sub.6 /(R.sub.5 +R.sub.6)+R.sub.6 [V.sub.B -R.sub.3 (V.sub.Z -V.sub.F)/R.sub.2 -V.sub.F ]/R.sub.4
where V.sub.Z is the constant voltage across the Zener diode 39 when it is conductive. Accordingly, the current i.sub.4 flowing through the grouped transistors 36 into the grouped transistors 37 is expressed as follows. EQU i.sub.4 =[V.sub.B -R.sub.3 (V.sub.Z -V.sub.F)/R.sub.2 -V.sub.F ]/R.sub.4
In this case, [R.sub.3 (V.sub.Z -V.sub.F)/R.sub.2 -V.sub.F ] is constant, so if the source voltage V.sub.S increases, the current i.sub.4 and the current i.sub.6 increases. As a result, the divided voltage V.sub.SS also increases to raise the base-emitter voltage of the transistor 35, increasing the base-emitter voltage of the transistor 33, so that current through the resistor 44 increases to raise the reference voltage V.sub.R.
When the shaped ignition signal V.sub.IR rises with the reference voltage V.sub.R thus set, it is fed through the resistor 46 to the base of the transistor 45 and turns it on. A current from the constant current supply 31 flows to ground via the now conductive transistor 45, turning off the transistor 33. Thus, the reference voltage V.sub.R is set by the constant current supply 41 and the transistor 43 whose base voltage is determined by the group of diodes 42.
As a result, the reference voltage V.sub.R in the form of an off-level reference voltage across the resistor 44 thus developed decreases so that the reference voltage V.sub.R as a whole has hysteresis, as shown by the chained line in FIG. 6, thus suppressing adverse influences of noise on ignition timings.
With the conventional ignition apparatus for an internal combustion engine as described above, the level of the ignition signal V.sub.I or the level of the reference voltage V.sub.R used for shaping the waveform thereof is varied in accordance with variations in the number of revolutions per minute of the engine and the source voltage of the power source such as the battery 5. For this purpose, the bias circuit 20, the on-level setting circuit 30 and the off-level setting circuit 40 are provided, and the electromagnetic pickup coil 1 is connected at one end thereof to the bias circuit 20 and at the other end thereof to the comparator 2. This results in a rather complicated circuit arrangement, an increased number of manufacturing steps, and a reduction in reliability.